1. Field of the Invention
This invention relates to an image-shake preventing device having correcting optical means such as a variable angle prism or the like.
2. Description of the Related Art
As a result of the recent advancement of automation of shooting apparatuses such as still cameras and video cameras, functions of varied kinds have been practicalized including, among others, automatic exposure adjustment means and automatic focusing means. In the field of video cameras, it is generally practiced to use a zoom lenses as a photo-taking lens. The zoom ratio of zoom lenses has been increasing year after year.
Meanwhile, efforts to reduce the size of the shooting apparatuses are showing salient results. The art of actually arranging a pickup image plane in a smaller size and in higher density has advanced. Various chassis have been developed for use with mechanisms of compact video recorders. As a result of these efforts, some of the recent products of shooting apparatuses have come to permit a shooting operation using one hand.
However, in operating such a compact video camera having a zoom lens, the vibration of the hand of the camera operator tends to cause an image shake to appear on the image plane. To obtain a stable image by eliminating such an image shake, various image-shake preventing devices have been proposed. These image-shake preventing devices not only eliminate the serious image shake due to the vibration of hands but, of course, also give a great advantageous effect even in a case where the image shake cannot be eliminated by the use of a tripod, like in the case of shooting on board of a ship or in a moving car.
Each of these image-shake preventing devices is composed at least of vibration detecting means for detecting a vibration and image-shake correcting means for making a correction according to information on the detected vibration so as to prevent an image shake from occurring on an image plane.
The vibration detecting means has been selected from among a group consisting of an angular acceleration meter, an angular velocity meter, an angular displacement meter, etc. As for the image-shake correcting means, either a variable angle prism is used in a manner as will be described in detail later herein or, in the case of a video camera in which a specific area of a pickup image plane is cut out from information on the pickup image plane and actually used as an image plane, the cutting out position of the area is arranged to be shifted to other positions where an image shake can be corrected one after another in such a manner as to track the image shake.
Hereinafter, the former image-shake correcting method of using optical means such as the above-stated variable angle prism for removal of an image shake in the stage of forming an image on an image sensor will be called "optical correcting means". The latter image-shake correcting method of removing an image shake through an electronic processing action on image information including the image shake hereinafter will be called "electronic correcting means".
Generally, the optical correcting means is capable of correcting an image shake resulting from a vibration taking place within a certain range of angles determined as an angular range of camera vibrations irrespective of the focal length of the lens of the camera. Therefore, even in the case a zoom lens having a long focal length on its telephoto side, the image-shake removing performance of the optical correcting means presents no problem for actual applications. The use of the optical correcting means, however, results in an increase in the size of the camera.
In the case of the electronic correcting means, on the other hand, the rate of correction on the image plane with respect to, for example, the vertical dimension of the image plane is fixed. Therefore, the image-shake preventing performance degrades accordingly as the focal length on the telephoto side increases. However, in many cases, the electronic correcting means is advantageous for reduction in the size of the camera.
FIGS. 15(A), 15(B), and 15(C) show a relation of the angle of vibration of a camera to the focal length through the position of an object image on an image plane.
Referring to FIG. 15(A), a lens has an optical axis 113 when the camera is in a position 112. In this case, the lens is directed to about the middle part of a person 111 which is an object of shooting. When, for example, the camera is caused to turn an angle of degrees "a" by the vibration of a hand, the camera comes to a position 114 where the lens has an optical axis 115.
FIGS. 15(B) and 15(C) indicate the positions of an image plane obtained respectively when the camera is in the positions 112 and 114. FIG. 15(B) shows a state obtained when a zoom lens is at its telephoto end position and FIG. 15(C) a state obtained when the zoom lens is at its wide-angle end position. An object image 116 is on an image plane. Image planes 117 and 119 are obtained when the camera is in the position 112. Image planes 118 and 120 are obtained when the camera is in the position 114.
As apparent from FIGS. 15(A), 15(B) and 15(C), with the camera vibrated at the same angle of degrees "a", the image is damaged by an image shake to a greater extent when the focal length of the zoom lens is longer. In a case where image-shake correcting means is to be arranged in combination with a lens having a long focal length at its telephoto end position, therefore, the use of the optical correcting means such as a variable angle prism is more advantageous than the electronic correcting means.
FIGS. 16(A), 16(B), and 16(C) show the arrangement of a variable angle prism. Referring to FIG. 16(A), reference numerals 121 and 123 denote glass plates. A bellows part 127 is made of a polyethylene material or the like. A transparent liquid 122 which is a silicone oil or the like is placed and sealed within a space encompassed with the glass plates 121 and 123 and the bellows part 127. In FIG. 16(B), the two glass plates 121 and 123 are shown in a parallel state. In this state, the incident angle and the exit angle of each ray of light incident on and exiting from the variable angle prism are equal to each other. However, when the two glass plates 121 and 122 are slanting at an angle to each other as shown in FIGS. 16(A) and 16(C), the ray of light is bent at some angle as indicated by lines 124 and 126.
Therefore, in a case where the camera is slanted by a vibration of the hand or the like, an image shake can be eliminated by controlling the angle of the variable angle prism which is disposed in front of the lens in such a way as to bend rays of light as much as the slanting angle.
FIGS. 17(A) and 17(B) show the state of such a control. In the case of FIG. 17(A), the two glass plates are in parallel with each other and a photo-taking optical axis is assumed to be directed to the head of the object. In the event of a vibration at an angle of degrees "a", the rays of light are bent, by driving the variable angle prism, an angle corresponding to the vibration, so that the optical axis can be kept directed to the head of the object, as shown in FIG. 17(B).
FIG. 18 shows by way of example a practicable arrangement of the variable angle prism which is provided with an actuator part arranged to drive the variable angle prism and an apex angle sensor arranged to detect the angular position of the variable angle prism.
Since actual variations take place in every direction. Each of front and rear glass surfaces is arranged to be rotatable on rotation axes which are in directions deviating 90 degrees from each other. All the component parts which are arranged in one of the two different rotating directions and indicated by reference numerals suffixed by "a" are arranged to function in the same manner as the functions of the component parts which are arranged in the other rotating direction and indicated by reference numerals suffixed by "b".
Referring to FIG. 18, the variable angle prism 141 consists of glass plates 121 and 123, a bellows part 127, a liquid 122, etc. The glass plates 121 and 123 are attached with an adhesive or the like respectively to holding frames 128a and 128b. The holding frames 128a and 128b respectively have rotation axes 133a and 133b in conjunction with fixed parts (not shown) and are thus arranged to be turnable around these axes 133a and 133b. The directions of these axes 133a and 133b differ 90 degrees from each other. On each of the holding frames 128a and 128b, a coil such as 135a or a corresponding element hidden from view is arranged in one body with the frame. Meanwhile, in a fixed part which is not shown, there is arranged a magnet such as 136a or a corresponding magnet hidden from view and yokes such as 137a or a corresponding yoke hidden from view and 138a or a corresponding yoke hidden from view. The arrangement is such that, when a current is allowed to flow to the coil 135a or the corresponding coil hidden from view, the variable angle prism 141 turns around the axis 133a or 133b. A slit 129a or a corresponding second slit hidden from view 129b is provided in the fore end of an arm part such as 130a or a corresponding arm part hidden from view which is formed in one body with the holding frame 128a or the corresponding holding frame hidden from view and extends from the holding frame 128a or the corresponding holding from hidden frame view. This slit 129a or the corresponding slit hidden from view is arranged to form an apex angle sensor in conjunction with a light emitting element such as 131a or a corresponding light emitting element hidden from view which is an infrared-emitting diode (IRED) or the like disposed on a fixed part and a light receiving element such as 142a or a corresponding light receiving element hidden from view which is a PSD (photosensitive diode) or the like.
FIG. 19 is a block diagram showing an image-shake preventing device which includes the above-stated variable angle prism 141 as image-shake correcting means and is arranged in combination with a lens.
Referring to FIG. 19, the illustration includes the variable angle prism 141, apex angle sensors 143 and 144, amplifier circuits 153 and 154 which are arranged to amplify respectively the outputs of the apex angle sensors 143 and 144, a microcomputer 145, vibration detecting means 146 and 147 respectively including angular accelerometers, etc., a lens 152, and actuators 148 and 149 each of which consists of the above-stated parts from the coil 135a or the corresponding coil hidden from view to the yoke 138a or the corresponding yoke hidden from view.
The microcomputer 145 is arranged to determine currents to be applied to the actuators 148 and 149 in controlling the variable angle prism 141 to an optimum angle position for removal of an image shake according to angle positions detected by the apex angle sensors 143 and 144 and the results of detection made by the vibration detecting means 146 and 147. In the arrangement shown in FIG. 19, each of the essential elements is shown in two blocks, because the control actions to be performed in the two different directions deviating 90 degrees are assumed to be carried out independently of each other.
The image-shake preventing device of the kind using the variable angle prism has been described above.
With the image-shake preventing device including such an optical correcting means that is arranged to move the variable angle prism and a part or the whole of an photo-taking lens, when the supply of currents to the actuators such as the coil 135a of FIG. 18 is cut off, for example, by turning off the main power supply of the camera, these actuators lose their powers for holding the variable angle prism or the movable lens within its movable range. Each of these moving parts then comes to a stop in a dynamically balanced position. However, this brings about the following problems:
(i) When an impact or vibration is applied from outside, the variable angle prism or the moving lens moves within its movable range. Then, if the moving extent is large, either an abnormal sound or a damage might be caused by a mechanical collision with a part located at a moving end.
(ii) With the variable angle prism used as the image-shake correcting means, the weight of the inside liquid might cause the two glass plates 121 and 123 which jointly form the variable angle prism as shown in FIGS. 16(A), 16(B) and 16(C) to become unable to keep their parallel state and to come to have some angle relative to each other. If the glass plates 121 and 123 are left in this state of having some angle either over a long period of time or under a high temperature and/or high humidity condition, the dynamic characteristic of the image-shake correcting means would be deteriorated.
To solve these problems, it is conceivable to keep a moving correcting optical member such as the variable angle prism or the moving lens approximately in the middle of their movable ranges by arranging a DC motor to transmit its driving force to a lever in association with the operation of the power supply. However, this arrangement necessitates the provision of a motor as an additional drive source solely for the purpose of moving a lever which is arranged to hold the optical correcting member approximately in the middle of its movable range. The use of such an additional drive source is undesirable in terms of reduction in size and energy saving.